I've often thought about this. Specifically, if you want to play a game of paintball or some other physical activity, there is often a waiver you sign which says something along the line of:

"you agree not to sue if you injure yourself, even if it was due to our negligence... "

When I sign it (because I can either agree, or not play the game of paintball with my mates), I just do so on the basis that a clause like that wouldn't hold up very well.